Yeast have five nonhomologous forms of double-stranded RNA (dsRNA) replicons called L-A, L-BC, T, W, and M. M encodes a secreted protein toxin and immunity to that toxin. We have extensively explored the genetics of replication of dsRNA viruslike entities in yeast. We have described 30 chromosomal genes needed for replication of the M dsRNA, three of which are also needed for L-A dsRNA. We also described six chromosomal genes whose products repress the replication of L-A, L-BC, and M dsRNAs. Current Findings: (A) We have detected a cytoplasmic genetic element, called [b], that elevates M copy number, making derepressed killers unable to grow normally and bypassing M's usual need for certain chromosomal gene products for its replication. (B) The chromosomal gene PET18 is involved in L-A--containing virus-like particle structural integrity. (C) We have detected new forms of M-containing virus-like particles and are examining their enzymatic activities. (D) We have cloned the SKI8, MAK11, MAK18, and MKT1 genes involved in M replication, and we have nearly completed the sequence of the MAK16 gene.